Add max context window model metadata (#18382)

Adds max_context_window to model metadata and routes core context-window
reads through resolved model info. Config model_context_window overrides
are clamped to max_context_window when present; without an override, the
model context_window is used.

codex-rs/core/tests/suite/rmcp_client.rs

@@ -1020,6 +1020,7 @@ async fn stdio_image_responses_are_sanitized_for_text_only_model() -> anyhow::Re
                 supports_parallel_tool_calls: false,
                 supports_image_detail_original: false,
                 context_window: Some(272_000),
+                max_context_window: None,
                 auto_compact_token_limit: None,
                 effective_context_window_percent: 95,
                 experimental_supported_tools: Vec::new(),